Bronchioles

Bronchioles are the branches of the tracheobronchial tree that by definition, are lacking in submucosal hyaline cartilage. 

Gross anatomy

The bronchioles typically begin beyond the tertiary segmental bronchi and are described as conducting, terminal or respiratory bronchioles. Following the tertiary segmental bronchi, there are 20-25 branching generations of conducting bronchioles which are typically <1mm in diameter. These transfer air but lack glands or alveoli with the most distal segment of these termed terminal bronchioles. 

Each of these terminal bronchioles themselves gives rise to several generations of respiratory bronchioles. These respiratory bronchioles are noted for thin-walled outpouchings from the lumen known as alveoli where primary gas exchange occurs. Thus, the respiratory bronchiole represents the first part of the respiratory division. Each respiratory bronchiole then supplies 2-11 alveolar ducts which in turn each supply 4-5 alveolar sacs. 

Histology

Bronchioles are defined by their lack of hyaline cartilage, instead relying on the tension from surrounding lung tissue for dilatory support. Despite this, smooth muscle is still present in the lamina propria of bronchioles and contributes to small airway closure in obstructive lung disease like asthma.

The larger bronchioles are lined with ciliated pseudostratified columnar cells typical of the classical respiratory epithelium. As further branching occurs, epithelial height lowers to produce more cuboidal ciliated cells. Additionally, the number of cilia and goblet cells also decrease until there are cilia free cuboidal cells found in the alveolar ducts.

Related pathology

• bronchospasm is the result of the lack of cartilage despite the presence of smooth muscle in bronchioles, acute onset of asthma and COPD can be life-threatening as a result of this small airway closure.
• bronchiolitis is an acute viral infection causing inflammation of the bronchioles and usually affects children less than two years of age. Typically the cause of Respiratory Syncytial Virus and can present with fever, respiratory distress and wheeze. Treatment is primarily supportive with oxygen and hydration.